Cork Institute of Technology. Autumn 2005 Manufacturing Engineering (Time: 3 Hours)

Transcription

1 Cork Institute of Technology Bachelor of Engineering (Honours) in Mechanical Engineering Stage 3 (Bachelor of Engineering in Mechanical Engineering Stage 3) Instructions Answer FIVE questions. Answer TWO from each of Sections A and B and ONE from Section C. Use a separate answer book for each section. (NFQ Level 8) Autumn 2005 Manufacturing Engineering (Time: 3 Hours) Examiners: Mr. M. P. O Mahony Dr. M. Woods Mr. D. Fallon Prof. J. Monaghan Mr. J. E. Hegarty Section A Q1. (a) List FIVE advantages and/or disadvantages of thread rolling. (5 Marks) (b) Briefly describe ONE method of Thread grinding. (5 Marks) (c) A special fine thread has a nominal outside diameter of 4 inches. The pitch is 0.25in and the threads are of Whitworth Form. The thread is acceptable if the effective diameter is between nominal size and minus in. When checked with wires of diameter 0.15in the dimensions over wires was in. If the flank angle and pitch are correct, calculate the effective diameter and hence state whether the thread is acceptable. (Note Height of fundamental triangle for Whitworth H= P) (8 Marks) (d) When measuring the effective diameter of a thread, explain what is meant by the term Best Wire? (2 Marks)

2 Q2. (a) An approximation to the work done in deforming a metal can be made, subject to two important assumptions. What are these assumptions? (4 Marks) (b) A billet of aluminum 25mm dia x 38mm long is compressed between flat parallel steel anvils to a length of 19mm. The initial yield stress is 65.4N/mm 2 and after 50% reduction the yield stress is 82.7N/mm 2. Find the frictionless work done in deforming the material and the mean force which would produce this amount of work. If µ = 0.14 find the maximum load to be exerted. (6 Marks) (c) List Three ways of countering the effects of Springback in a bending operation. (3 Marks) (d) Fig Q2 (below)shows the dimensions of a bracket required to be bent from a flat strip of mild steel of 450N/mm 2 yield stress. If the work is drawn to a depth of 18mm below the top surface of the die, find the punch load required for: (i) Bending (1 Mark) (ii) Overcoming friction, assuming the sides of the component are very slightly ironed, take µ=0.15 (2 Marks) (iii) Planishing at the end of the stroke (2 Marks) (e) List TWO rules that should be followed in the design of piercing and blanking dies. (2 Marks) Figure 2 Q3. (a) In the context of Casting describe the purpose and use of the following: Chaplet Chill Riser (6 Marks) (b) Using diagrams, describe FOUR casting defects. 8 Marks) (c) A cylindrical riser must be designed for a sand casting mold. The casting itself is a steel rectangular plate with dimensions 3.0 x5.0 x1.0. The t s for this casting is 1.6min. The cylinder for the riser will have a D/h ratio = 1.0. Determine the dimension of the riser so that it s t s is 25% more than the casting. Take n=2. (6 Marks)

3 Section B Q4. (a) (i) Describe the fast fracture micro-mechanism for metals. What difference is there in the micro-mechanism between hard and soft metals? Use diagrams to illustrate your answer. (ii) Compare the difference in the appearance under a microscope of a material failing by fast fracture by the ductile tearing process with that of the cleavage process. (6 marks) (b) A pin is inserted into partially inflated and fully inflated balloons. Explain why the partially inflated balloon does not burst, whereas the fully inflated balloon bursts by fast fracture. State the energetic conditions for a tear to propagate when a balloon bursts. (4 marks) (c) State the fast fracture condition. Define each of the parameters in the equation. An aluminium, alloy component has a fracture toughness K c = 23 MN m -3/2. What is the maximum crack size that can be tolerated at the surface before fast fracture occurs when a stress of 125 MN m -2 is applied? (6 marks) (d) Describe the mechanism for fatigue in a metal. (4 marks) Q5. (a) What is diffusion? Describe how substitutional [vacancy] diffusion occurs for in metals. What are the effects of temperature and activation energy on diffusion? (8 marks) (b) (i) State the solution to Fick s Second Law. What does this law describe? (ii) A carburising process is performed on 0.15% steel by introducing 1.1% C to the surface at 1000 o C. Calculate the carbon content at a depth of 0.5 mm after 1 hour. The diffusivity of C in steel D = 5.0 x m 2 s -1. (Error function tables are provided) (8 marks) (c) Describe the sintering process. (4 marks)

4 Q6. (a) Explain the origins of friction between metal surfaces. Explain how the use of a lubricant oil reduces friction. (5 marks) (b) (i) Describe the process of adhesive wear for two different materials (A and B) in contact for the case of A and B having equal hardness and also differing hardness. What happens if one of the materials has a case-hardened surface? (ii) Describe abrasive wear of materials. Give ONE example of an application where abrasive wear is desirable. (iii) Explain how wear may be reduced. (10 marks) (c) Sketch the creep curve for a material under constant load at constant temperature. Briefly describe the temperature requirement and the creep mechanism for power law creep in region 2 of the curve. (5 marks)

5 Section C: Quality Engineering Q7. (a) A process which is in control is not necessarily capable! Discuss whether or not this statement is true. (6 Marks) (b) The data shown below are the sample mean and ranges of deviations from nominal diameter for holes drilled in a carbon fiber composite material used in aerospace manufacturing. The values reported are deviations in ten thousandths of an inch and the sample size is five. (i) Using the blank control charts and tables of constants provided, set up X and R charts on the process. Is the process in statistical control? (8 Marks) (ii) Estimate the process standard deviation using the range method. (2 Marks) (c) If specifications are at nominal ± 100, what can you say about the capability of the process? Calculate C p and C pk indices for the process. (4 Marks) Table 8.1 Sample X R Sample X R

6 Q8.(a) Discuss briefly whether or not improving Quality is expensive. (4 Marks) (b) One of Deming s 14 points urges that organisations cease dependence on inspection to achieve quality. Eliminate the need for mass inspection. Discuss this assertion in detail in the context of quality in manufacturing. (10 Marks) (c) Discuss what is meant by the Quality Function within an organization, discussing who is responsible for the Quality of products and how that responsibility is met. (6 Marks)

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